A turbofan gas turbine engine operates according to well known principles wherein an incoming air stream flows through the engine along an annularly configured, axially extending flow path. A portion of the incoming air stream is compressed in a compressor section of the engine and then mixed with fuel and burned in a combustor section to produce a high energy, high temperature exhaust gas stream. The gas stream exits the combustor and subsequently passes through a turbine section that extracts energy from the exhaust gas stream to power the compressor and produce bypass thrust by rotating a fan that acts generally on the remaining portion of the incoming air stream.
Uncontrolled leakages of gases within the engine contributes to a reduced engine efficiency. Seals are used to control this energy loss by interposing them in a leakage path to reduce the volume or mass of the gas--atmospheric air, exhaust, or otherwise--passing from one part of the engine to the other. In the past engine seals have principally taken the form of labyrinth seals. The use of brush seals as a substitute for labyrinth seals is presently being investigated.
A typical brush seal includes a plurality of seal stages with each stage including a bristle pack having a plurality of bristles of a substantially uniform cross sectional area. The bristle pack is disposed between a pair of annularly configured plates. Usually the bristles are disposed at about a forty five degree angle to a radius drawn from the engine center line. A brush seal is usually attached along its outer circumferential edge to a stationary portion of the engine with the inward, free ends of the bristles disposed in a sealing engagement with a sealing surface on a rotating engine part. Brush seals are not intended to function so as to completely seal one engine part from another, but rather rely upon the torturous flow path created between the bristles to reduce the airflow from one part of the engine to another and to control the pressure drop between the engine parts.
The bristles in the bristle pact are somewhat flexible; thus, they are able to bend during an engine transient and still retain their sealing ability after the transient has passed. Examples of such transients include differential thermal growth between the engine parts, rotor/stator relative movement, and vibration of some sort. Thus, a rotating engine shaft, for example, may enter a vibration mode where the shaft is vibrating about its longitudinal axis, that is, when the shaft is rotating eccentrically.
The sealing efficiency of a brush seal over time is affected by the wear on the bristle ends contacting the sealing surface on the opposing engine part, as well as the overall contact of the bristle ends with the sealing surface. Worn bristles ends will dictate replacement of the seal or particular seal stage earlier than otherwise would be necessary, thereby increasing engine operating costs. Eccentric rotation of a rotor shaft can create such unwanted bristle wear.
Eccentric shaft rotation has been found to induce a one per revolution unsteady flow with respect to the stationary bristles. This unsteady flow is the result of a "blow-by" situation being created by the eccentric shaft rotation, which causes a gap to open between the bristle ends and the rotating engine part. The unsteady flow causes the bristles to vibrate as a cantilever beams with the free ends of the bristles deflecting radially, tangentially, and axially. Radial deflection of the free ends inwardly increases the rubbing force between the free ends of the bristles and the sealing surface, thereby causing the free ends to wear. Because the amplitude of the vibration decreases axially from the inlet or upstream side of the seal to the outlet or downstream side of the seal, the upstream bristle free ends experience wear to a greater extent than the downstream free ends. Thus the bristles are chamfered by the one per revolution induced vibration.
It would be desirable to increase the lifetime and sealing efficiency of brush seals by reducing chamfering of the bristle pack caused by one per revolution induced vibrations during eccentric rotation of a rotor shaft.